Where did those brown leaves come from this time of the year? The offending tree in the background is in full green leaf. XX

It's a bloody LIME tree...What do you expect?

Or have you never seen one before?

I will put this to you gently, THAT is how they spread their seeds. The leaves of the pod go brown, and drop off, acting like a cross bwtween a helicopter and a parachute. This normaly happens from mid June until Septemberish, when the main leaves start browning.

As to the size of the tree, from that angle you can make a bastard bonsai look like a California red wood!